Semiconductor device and method for manufacturing the same

ABSTRACT

In the present invention, provided are a semiconductor device having a semiconductor-element-mounting substrate on which a semiconductor element has been mounted via an adhesive having an exothermic-reaction curing start temperature of 130° C. or below as measured with a differential scanning calorimeter at a heating rate of 10° C./minute, and a process for its fabrication.

TECHNICAL FIELD

This invention relates to a semiconductor device and a process for itsfabrication.

BACKGROUND ART

In recent years, as a trend of semiconductors, their packaging in ahigher density is being advanced. With this trend, the mainstream ofsemiconductor device packaging has shifted from through-hole mountpackaging to surface-mount packaging. Also, the number of input/outputterminals of a package is being made larger with an improvement in thedegree of integration of semiconductors.

Hitherto, as typical surface-mount type semiconductor devices, QFP (quadflat package), for example, has been used which is a structure in whichsemiconductor elements are mounted on a metallic lead frame, the wholeof the semiconductor element is encapsulated after gold wire bonding iscarried out, and outer leads are cut and shaped and are put out from thesides of an encapsulated portion. However, in order to make thisstructure have terminals in a larger number, the terminal pitch must bemade smaller. In a region with a pitch of 0.5 mm or smaller, ahigh-level technique is required for their connection to a mother boardon which semiconductor elements are to be mounted. Accordingly, OMPAC(over-molded pad grid carrier) type BGA (ball grid array) whereouter-face devices are arranged in array has been developed, and isbeing put into practical use.

This BGA enables arrangement of external terminals in a large number perunit area, and allows easy face-bonding mount onto the mother board,also enabling miniaturization with ease in comparison with theabove-mentioned QFP.

An example of this OMPAC type BGA (hereinafter simply “BGA”) is shown inFIG. 2. First, a semiconductor element 1 is fixed onto asemiconductor-element-mounting substrate 14 by means of an adhesive 3 orthe like. Terminals on the semiconductor element 1 are electricallyconnected with gold-plated terminals 7 formed on thesemiconductor-mounting substrate through gold wires 8. The semiconductorelement 1 is further encapsulated with an organic insulating encapsulant5. On an insulating base material 2, a metallic wiring pattern isincorporated which is electrically interconnected with a semiconductorelement electrode. This metallic wiring pattern is constituted of finewiring patterns 6 for transmitting electric signals. These are connectedwith external connecting terminals 9 through the insulating basematerial 2 on the back which is opposite to the side on which thesemiconductor element is to be mounted. Also, in many cases, the finewiring patterns 6 and the insulating base material 2 are covered with aninsulating protective resist 4 at areas except for those of the regionwhere the gold-plated terminals 7 are arranged for wire bonding and ofthe external terminals 9. Meanwhile, external terminals provided withsolder balls in array are formed on the substrate. These are connectedwith external connecting terminals 9 through the insulating basematerial 2 on the back which is opposite to the side on which thesemiconductor element is to be mounted. Also, in many cases, the finewiring patterns 6 and the insulating base material 2 are covered with aninsulating protective resist 4 at areas except for those of the regionwhere the gold-plated terminals 7 are arranged for wire bonding and ofthe external terminals 9. Meanwhile, external terminals provided withsolder balls in array are formed on the substrate.

In such a BGA device, members that occupy the greater part of the BGA'swhole volume, except for those of the semiconductor element 1, finewiring patterns 6, gold wires 8, gold-plated terminals 7 and solderballs 10, are organic materials, and hence these members absorb moistureduring the storage of the device. For this reason, for example, as shownin FIG. 2, the moisture absorption water content may vaporize at a gap13 formed between the semiconductor element 1 and the adhesive 3 tocause cracking or separation 11 in the interior of the insulatingprotective resist 4 embraced in the semiconductor-mounting substrate, orat the interface between the insulating protective resist 4 and theorganic insulating encapsulating member 5 (at the interface between theinsulating base material and the organic insulating encapsulating member5 when the insulating protective resist 4 is not provided). Then, wherethis cracking or separation 11 has extended to the gold-plated terminals7 for wire bonding as shown in FIG. 2, it may cause faulty electricalinterconnection in the worst case.

DISCLOSURE OF THE INVENTION

The present invention provides a semiconductor device which may lesscause cracking when mounted by soldering and can improve electricalinterconnection reliability.

The semiconductor device of the present invention comprises;

(A) a semiconductor-element-mounting substrate having an insulating basematerial on which a stated wiring pattern connected electrically with asemiconductor element electrode has been formed;

(B) a semiconductor element bonded to the semiconductor-element-mountingsubstrate via an adhesive and connected electrically with the wiringpattern; and

(C) an organic insulating encapsulant with which the semiconductorelement is encapsulated at least at its electrode portion;

(D) the adhesive having an exothermic-reaction curing start temperature(Th) of 130° C. or below as measured with a differential scanningcalorimeter (DSC) at a heating rate of 10° C./minute.

In the semiconductor device of the present invention, as thesemiconductor-element-mounting substrate (A), used is asemiconductor-element-mounting substrate having an insulating basematerial on which a stated wiring pattern connected electrically with asemiconductor element electrode has been formed and having an externalconnecting terminal conducting to the wiring pattern, formed on the backwhich is opposite to the side on which the wiring pattern has beenformed, and the organic insulating encapsulant (C) can be so applied asto encapsulate the whole of the semiconductor element (i.e., so appliedthat the semiconductor element surface is not uncovered).

The adhesive (D) may also preferably have a saturation moistureabsorption of 0.18% by weight or less at 30° C. and 85% RH.

The present invention also provides a process for fabricating asemiconductor device; the process comprising the steps of;

bonding via an adhesive a semiconductor element to the surface of asemiconductor-element-mounting substrate having an insulating basematerial on which a wiring pattern has been formed, to connect anelectrode of the semiconductor element electrically with the wiringpattern; and

encapsulating with an organic insulating encapsulant the semiconductorelement at least at its electrode portion;

the adhesive comprising an adhesive having an exothermic-reaction curingstart temperature of 130° C. or below as measured with a differentialscanning calorimeter at a heating rate of 10° C./minute.

The semiconductor device having the construction that the externalterminals solder balls 10 are taken out from the surface of thesemiconductor-mounting substrate as stated previously (FIG. 2) has anadvantage that the multi-pin structure can be achieved with ease.However, it on the other hand has a problem that a warpage deformationtends to occur which starts from the center of the semiconductor devicewhen it is cooled from molding temperature to room temperature or whenthe temperature is raised to reflow temperature, because of the factthat the semiconductor device has the construction of one-sideencapsulation in shape and because of a great difference in values ofphysical properties between the semiconductor-element-mounting substrateand the organic insulating encapsulant 5.

Hence, a plurality of solder balls 10 having been so arranged as to beon the same plane on the semiconductor-mounting substrate come not to bearranged on the same plane as the semiconductor device undergoes awarpage deformation, to become different in height at some part. Whenthis is tested in a packaging inspection step, a difficulty may arise inthe connection of connectors to cause a trouble that no sufficientinspection can be made. Also, when such a semiconductor device issurface-mounted on a printed circuit board, some balls can not perfectlybe connected to their corresponding wiring layers in the worst case,resulting in a low reliability at connection areas in some cases.

Accordingly, in the present invention, as the organic insulatingencapsulant, it is preferable to use an encapsulant having a glasstransition temperature between molding temperature and room temperature,having a difference of 0.6×10⁻⁵/° C. or above between the coefficient oflinear expansion of the semiconductor-element-mounting substrate and thecoefficient of linear expansion of the organic insulating encapsulant ata temperature not higher than the glass transition temperature, andhaving a curing shrinkage factor of 0.11% or less at the time of themolding of the organic insulating encapsulant.

Use of such an encapsulant can make cracking less occur when mounted bysoldering and also can improve electrical interconnection reliability,preventing the semiconductor device from causing warpage deformation.

The organic insulating encapsulant may preferably have a saturationmoisture absorption of 0.36% by weight or less at 85° C. and 85% RH. Theorganic insulating encapsulant may also preferably have a modulus inflexure of 4.0 GPa or below at the molding temperature of the organicinsulating encapsulant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration of the construction of thesemiconductor device of the present invention.

FIG. 2 is a cross-sectional view of a BGA semiconductor device havingcaused cracking at the time of reflowing.

FIGS. 3A-3D are cross-sectional illustrations of semiconductor devicefabrication steps where a semiconductor element is mounted on asemiconductor-mounting substrate.

FIG. 4 is a graph showing the relationship between temperature andcalorific value as measured with a differential scanning calorimeter(DSC), which represents curing behavior of an adhesive.

FIG. 5 is a perspective view of a sample used to measure the saturationmoisture absorption of the adhesive.

FIG. 6 is a graph showing the relationship between the temperature andthe extent of shrinkage of an organic insulating encapsulant.

BEST MODES FOR PRACTICING THE INVENTION

As a result of detailed examination of a condition where the cracking orseparation occurs when mounted by soldering, it has been found that suchcracking or separation is chiefly caused by the water content absorbedas moisture and diffused from the back of thesemiconductor-element-mounting substrate and from the organic insulatingencapsulant and that this water content is easily accumulated in thevoids of an adhesive layer to tend to cause the cracking or separation.

FIGS. 3A to 3D show semiconductor device fabrication steps where asemiconductor element 1 is mounted on the semiconductor-mountingsubstrate.

As shown in FIG. 3A, a semiconductor-mounting substrate 14 to beprocessed absorbs water content 12 as moisture from the atmosphere whileit is left. Where a semiconductor element 1 is bonded to this substrate14 via an adhesive 3 [FIG. 3B] and the device being in this step isheated in order to cure the adhesive 3, the water content 12 adsorbed asmoisture from the surface of the semiconductor-mounting substratevaporizes as shown in FIG. 3C. This water content 12 may enter theadhesive 3 layer before the adhesive is begun to cure, so that voids 13are formed in a large number as shown in FIG. 3D.

Where the semiconductor device is fabricated using as it is the onehaving the voids 13 formed in the adhesive 3 layer, water contentseparately absorbed as moisture and diffused from the back of thesemiconductor-mounting substrate and from the organic insulatingencapsulant 5 may easily be accumulated in the voids 13 of the adhesive3 layer. As the result, the cracking or separation 11 comes to tend tooccur as shown in FIG. 2. The present invention is based on such a newfinding.

FIG. 3A illustrates how the substrate 14 absorbs the water content 12 asmoisture from the atmosphere while the semiconductor-mounting substrate14 is left in the atmosphere. FIG. 3B is a cross-sectional view of thesemiconductor-mounting substrate 14 on which the semiconductor element 1has been mounted via the adhesive 3. FIG. 3C illustrates that the watercontent vaporizes from the surface of the semiconductor-mountingsubstrate 14 and enters the adhesive 3 layer while the substrate onwhich the element has been mounted, shown in FIG. 3B, is heated in anoven. FIG. 3D is a cross-sectional view showing that the voids 13 havebeen formed in the adhesive 3 layer after the heating step shown in FIG.3C has been completed.

As stated above, the water content absorbed as moisture in thesemiconductor-mounting substrate 14 comes to be the cause of forming thevoids 13 in the adhesive 3 layer. Now, the values of moisture absorptionof a semiconductor-mounting substrate 14 commonly used in an atmosphereof 30° C. and 60% RH are shown in Table 1. Here, thesemiconductor-mounting substrate 14 is a substrate comprising aninsulating base material which is constituted of an epoxy resin and aglass cloth and on both sides of which fine wiring patterns are providedand are further covered thereon with an insulating protective resist.

As can be seen from Table 1, the semiconductor-mounting substrate usedin BGA packaging begins to absorb moisture rapidly, and its moistureabsorption reaches more than 0.1% by weight in few hours or so. When inthis stage the adhesive is applied and heated to cure in the same manneras that in conventional metallic-frame semiconductor devices (QFP and soforth), the water content vaporizes from the surface of thesemiconductor-mounting substrate having absorbed as moisture the watercontent in the atmosphere, to enter the adhesive layer, and there is apossibility of causing voids in a large number.

TABLE 1 Substrate moisture absorption in atmosphere of 30° C./60%RHMoisture Moisture absorption absorption Time lapsed (wt. %) Time lapsed(wt. %) 1 hour  0.06 6 hours 0.13 2 hours 0.09 8 hours 0.15 4 hours 0.1216 hours  0.18

Accordingly, in the present invention, the curing start temperature ofthe adhesive is appropriately set so that the adhesive can be cured at alow temperature before the water content having vaporized from thesurface of the semiconductor-mounting substrate enters the adhesivelayer, and thus the adhesive layer can be prevented from causing voids.More specifically, in the present invention, an adhesive having anexothermic-reaction curing start temperature (Th) of 130° C. or below asmeasured with a differential scanning calorimeter (DSC) at a heatingrate of 10° C./minute is used as the adhesive.

Here, the curing start temperature (Th) indicates, as shown in FIG. 4,the temperature in the part of a reaction region of a DSCtemperature/heat flow rate curve and at which the curve starts to riseabruptly, immediately before it reaches the maximum rate of heat flow.The adhesive starts to react and cure abruptly, from the point of timeit has reached this temperature. Also, setting the Th to 130° C. orbelow not only enables achievement of the effect of less causing thevoids, but also promises a superior short-time curability, and hencethis is very effective also for improving production efficiency.

The adhesive used in the present invention may be of any types withoutany particular limitations as long as it has the curing performancedescribed above. In order to keep the voids from being caused by theevaporation of a solvent contained in the adhesive, the adhesive maypreferably be comprised of a base resin having a low viscosity and beingof a solvent-free type. As the base resin, usable are acrylic resins,for example.

In order to control the quantity of water content accumulated in theadhesive layer after the semiconductor device has been fabricated, it ismore preferable to use as the adhesive an adhesive having a saturationmoisture absorption of 0.18% by weight or less under conditions of 30°C. and 85% RH.

Here, the saturation moisture absorption is measured in the followingway: As shown in FIG. 5, an adhesive 52 to be measured is put betweentwo sheets of sheet glass 51 of 18 mm long, 18 mm wide and 0.2 mm thickand with flat surfaces, and four sheets of aluminum foil of 3 mm long, 3mm wide and 0.03 mm thick are used as spacers 53. In FIG. 5, the scalein thickness direction is arbitrarily altered so as to be seen withease.

First, the mass (weight) of the sheet glass 51 and that of the aluminumfoil (spacers 53) are accurately measured up to a figure of 0.01 mg.Next, 50 to 60 mg of the adhesive 52 is put between the two sheets ofsheet glass 51 together with the spacers 53, and then cured at 180° C.for 1 hour. The mass of the resultant test piece is accurately measuredup to a figure of 0.01 mg. Thereafter, in an atmosphere of 30° C. and85% RH, the test piece is left to absorb moisture up to the lapse of3,000 hours on which its mass comes to little change. The mass of thetest piece after moisture absorption is accurately measured up to afigure of 0.01 mg. The saturation moisture absorption is a value SAcalculated according to the following equation (1).

SA={(V ₂ −V ₁ −V ₀)/(V ₁ −V ₀)}×100  (1)

SA: Saturation moisture absorption of the adhesive.

V₀: Mass of plate glass and aluminum foil.

V₁: Mass of the test piece before moisture absorption.

V₂: Mass of the test piece after moisture absorption.

The adhesive may preferably be those having an adhesive force as high aspossible in order to prevent the adhesive from separating on the back ofthe semiconductor element or at the interface with thesemiconductor-mounting substrate because of the vaporization andexpansion of water content which are caused at the time of reflowing. Inparticular, those which do not greatly decrease in adhesive force as aresult of moisture absorption are preferred.

The present inventors made further detailed examination on a conditionwhere the warpage deformation occurs in the semiconductor device, andhave discovered that the warpage deformation can be made less occur whenan encapsulant having specific properties is used as the organicinsulating encapsulant 5. The effect of making this warpage deformationless occur is explained below.

First, to elucidate the mechanism by which the warpage deformationoccurs in BGA packaging, an example of a shrinkage curve of an organicinsulating encapsulant at temperatures of from molding temperature toroom temperature is shown in FIG. 6. Where an organic insulatingencapsulant is heated in a mold to effect curing and then released fromthe mold to leave it cool, first the organic insulating encapsulantreacts to cure at molding temperature. At this stage, a shrinkage causedby the curing reaction (hereinafter “curing shrinkage a.”) occurs. Next,when the molded product is released from the mold to leave it cool toroom temperature, a shrinkage based on linear expansion of the organicinsulating encapsulant (hereinafter “heat shrinkage b.”) occurs. Ashrinkage put together by adding the curing shrinkage a. and the heatshrinkage b. is total shrinkage c. of the organic insulatingencapsulant.

Now, where the organic insulating encapsulant is molded on asemiconductor device of one-side encapsulation structure like BGA andthere is a great difference between the total shrinkage c. of theorganic insulating encapsulant and the coefficient of linear expansionof the semiconductor element and semiconductor-mounting substrate whichare adjoining thereto, a residual stress is produced in the interior ofthe semiconductor device to cause warpage deformation over the wholesemiconductor device.

In general, the warpage deformation of laminated structures occurs as aresult of accumulation of residual stress in the interior of thestructures. Hence, in order to reduce the warpage deformation, it iseffective to relax the residual stress in the interior of thesemiconductor device. Also, in order to relax the residual stress, it iseffective to lower the modulus of elasticity of constituent members. Inparticular, it is effective for the organic insulating encapsulant tohave a wide region of lowering of the modulus of elasticity inviscoelastic behavior within a temperature range of from roomtemperature to molding temperature, i.e., to have a glass transitiontemperature within this temperature range. With such a feature, itfollows that the temperature region in which the modulus of elasticityof the organic insulating encapsulant lowers in a wide range is embracedin the temperature that is in the course of cooling to room temperatureafter package molding or in the course of raising temperature from roomtemperature in the step of solder reflowing. Thus, the residual stressproduced in the device can be relaxed and the warpage deformation caneffectively be made to less occur.

Accordingly, in the present invention, it is preferable to appropriatelyuse as the organic insulating encapsulant an encapsulant having a glasstransition temperature in the temperature range of from room temperatureto molding temperature. This is because the use of such an encapsulantenables the warpage deformation to less occur on the level ofsemiconductor devices. Incidentally, the lower limit of the modulus ofelasticity thus lowered may preferably be about {fraction (1/10)} of themodulus of elasticity at room temperature. Molding temperature maypreferably be within the range of from 170 to 180° C., but is notparticularly limited thereto as long as it is within a temperature rangein which the molding properties of the organic insulating encapsulantare not damaged.

The substrate and the encapsulant may also be used in appropriatecombination so that the semiconductor-mounting substrate and the organicinsulating encapsulant may have a difference in heat shrinkage within astated range. This enables relaxation of the residual stress to makesmall the warpage deformation on the level of semiconductor devices.

As a result of extensive studies, the present inventors have discoveredthat, taking account of the influence of the semiconductor elementmounted on the inner-wall side of the organic insulating encapsulant andthe extent of total shrinkage of the curing shrinkage and heat shrinkageput together, the organic insulating encapsulant may preferably have adifference in coefficient of linear expansion, of 0.6×10⁻⁵/° C. or abovebetween the semiconductor-mounting substrate and the organic insulatingencapsulant at a temperature not higher than the latter's glasstransition temperature. This can effectively lower the extent of warpagedeformation at the time of the cooling from molding temperature to roomtemperature.

The present inventors have also discovered that the use of an organicinsulating encapsulant having a specific value of extent of curingshrinkage at the time of molding can make small the warpage deformationon the level of semiconductor devices. Accordingly, in the presentinvention, it is preferable to use an organic insulating encapsulanthaving a curing shrinkage factor of 0.11% or less.

An encapsulant which undergoes curing shrinkage at an appropriate extentnot only can make the total shrinkage of the organic insulatingencapsulant appropriate in extent against the warpage deformation causedwhen it is cooled from molding temperature to room temperature, but alsocan make the warpage deformation as small as possible at the temperature(in general, around molding temperature of about 170-180° C.) at whichthe solder begins to melt at the time of solder reflowing, to contributeto an improvement in the reliability of connection with a substrate formounting.

Here, the curing shrinkage factor is a value dP obtained in thefollowing way: A test piece constituted of a single member of 60.1 mmlong, 6 mm wide and 1.5 mm thick is molded, and fist the molding cavitysize in the longitudinal direction at molding temperature and the testpiece size in the longitudinal direction at room temperature immediatelyafter molding are accurately measured with a precision slide caliper upto a figure of 0.01 mm, and a shrinkage factor dL of the whole iscalculated according to the following equation (2). Thereafter, a heatshrinkage factor dT at temperatures of from molding temperature to roomtemperature separately measured is subtracted from this value dLaccording to the following equation (3) to calculate the value dP.

dL=(L ₀ −L)/L ₀  (2)

dL: Molding shrinkage factor of the whole.

L₀: Molding cavity size at molding temperature.

L: Test piece size at room temperature.

dP=dL−dT  (3)

dP: Curing shrinkage factor.

dL: Molding shrinkage factor of the whole.

dT: Heat shrinkage factor.

In the present invention, in order to control the quantity of watercontent diffused from the organic insulating encapsulant to the adhesivelayer, it is preferable to use an organic insulating encapsulant havinga saturation moisture absorption of 0.36% by weight or less at 85° C.and 85% RH.

Here, the saturation moisture absorption is a value obtained in thefollowing way: A disk (according to JIS-K6911) constituted of a singlemember of 50 mm diameter and 3 mm thick is dry-treated at 120° C. for 2hours as pretreatment, and then its mass (weight) is accurately measuredup to a figure of 1 mg. Thereafter, in an atmosphere of 85° C. and 85%RH, the test piece is left to absorb moisture up to the lapse of 3,000hours on which its mass comes not to change any longer. The mass of thetest piece after moisture absorption is accurately measured up to afigure of 1 mg. The saturation moisture absorption is a value SMcalculated according to the following equation (4).

SM={(W ₂ −W ₁)/W ₁}×100  (4)

SM: Saturation moisture absorption of the organic insulatingencapsulant.

W₁: Mass of the test piece before moisture absorption.

W₂: Mass of the test piece after moisture absorption.

In the step of solder reflowing, in order to further relax the residualstress in the interior of the semiconductor device when connected with aprinted circuit board, the organic insulating encapsulant may alsopreferably have a modulus in flexure of 4.0 GPa or below at atemperature at which the solder begins to melt, i.e., about the moldingtemperature. Here, the modulus in flexure is a value obtained by makinga three-point bending test on a test piece (according to JIS-K6911)constituted of a single member of 70 mm long, 10 mm wide and 3 mm thick,and calculated according to the following equation (5).

E=(I·ΔP)/(4wh·Δy)  (5)

E: Modulus in flexure I: Span

ΔP: Load w: Lateral length of the test piece

Δy: Displacement h: Height of the test piece

As the base resin of the organic insulating encapsulant, those composedchiefly of an epoxy resin having a biphenyl skeleton are preferred inview of an advantage that the effect of especially great viscoelasticproperties is obtainable within a temperature range of from moldingtemperature to room temperature. In the organic insulating encapsulant,a material having a rigid structure that can make any free volumetricchange small after curing may preferably be mixed for the purpose ofreducing the curing shrinkage factor. A filler mixed in the organicinsulating encapsulant may preferably be in an amount not less than 80%by volume in view of the controlling of heat shrinkage factor andmoisture absorption.

As the insulating base material, usable are, e.g., organic insulatingbase materials. Composite materials comprised of a reinforcing materialsuch as glass cloth impregnated with a synthetic resin such as epoxyresin, polyimide or phenolic resin may be used, and besides syntheticresin films such as TAB (tape automated bonding) tape materials maybeused. As the insulating base material, those having a moistureabsorption as low as possible may preferably be used so that any voidscan be kept from being caused in the adhesive layer in the course of thecuring of the adhesive and also the water content can be kept from beingdiffused from this base material member to the adhesive layer after thefabrication of semiconductor devices.

The present invention will be described below in a specific manner bygiving Examples. The present invention is by no means limited to these.

EXAMPLE 1

A semiconductor-element-mounting substrate having an external size of26.2 mm long, 26.2 mm wide and 0.6 mm thick having, as shown in FIG. 1,an insulating base material 2 (a glass cloth-epoxy resin laminated sheetavailable from Hitachi Chemical Co., Ltd.; trade name: E-679) on whichfine wiring patterns 6 were formed and an insulating protective resist 4(available from Taiyo Inki K. K.; trade name: PSR4000 AUS5) was coatedon the surface except for gold-plated terminals 7 and opposite-sideexternal connecting terminals 9, was dried at 120° C. for 2 hours andthen left for 5 hour in an atmosphere of 30° C. and 60% RH. Thereafter,a semiconductor element 1 of 9 mm long, 9 mm wide and 0.51 mm thick wasmounted thereon, coating an adhesive 3 (available from Hitachi ChemicalCo., Ltd.; trade name: EN-X50), followed by heating in an clean oven for1 hour from room temperature to 180° C. at a constant heating rate, andfurther followed by heating for 1 hour at a constant temperature of 180°C.

Thereafter, wire bonding portions and semiconductor element electrodeswere wire-bonded through gold wires 8 of 30 μm in diameter. Next, usingan organic insulating encapsulant 5 (available from Hitachi ChemicalCo., Ltd.; trade name: CEL-X9600), the semiconductor element mountedarea was encapsulated by transfer molding under conditions of 175° C.,90 seconds, and 6.9 MPa, followed by post-curing under conditions of175° C. and 5 hours to obtain a BGA semiconductor device.

EXAMPLE 2

A BGA semiconductor device was obtained in the same manner as in Example1 except that a different adhesive (available from Hitachi Chemical Co.,Ltd.; trade name: EN-X52) was used.

EXAMPLE 3

A BGA semiconductor device was obtained in the same manner as in Example1 except that CEL-9000 (trade name), available from Hitachi ChemicalCo., Ltd., was used as the organic insulating encapsulant. In thepresent Example, the difference between the coefficient of linearexpansion of the semiconductor-element-mounting substrate and thecoefficient of linear expansion of the organic insulating encapsulant ata temperature not higher than the glass transition temperature is lessthan 0.6×10⁻⁵/° C.

EXAMPLE 4

A BGA semiconductor device was obtained in the same manner as in Example1 except that CEL-7700SX (trade name), available from Hitachi ChemicalCo., Ltd., was used as the organic insulating encapsulant. Theencapsulant used in the present Example had a curing shrinkage factormore than 0.11% and a saturation moisture absorption of more than 0.36%by weight at 85° C. and 85% RH.

EXAMPLE 5

A BGA semiconductor device was obtained in the same manner as in Example1 except that CEL-1731 (trade name), available from Hitachi ChemicalCo., Ltd., was used as the organic insulating encapsulant. Theencapsulant used in the present Example had a glass transitiontemperature at the molding temperature or above, where the differencebetween the coefficient of linear expansion of thesemiconductor-element-mounting substrate and the coefficient of linearexpansion of the organic insulating encapsulant at a temperature nothigher than the glass transition temperature is less than 0.6×10⁻⁵/° C.The encapsulant had a modulus in flexure above 4.0 GPa at its moldingtemperature, and a saturation moisture absorption more than 0.36% byweight at 85° C. and 85% RH.

EXAMPLE 6

A BGA semiconductor device was obtained in the same manner as in Example1 except that an adhesive (available from Hitachi Chemical Co., Ltd.;trade name: EN-4570) having a saturation moisture absorption of 0.18% byweight or more at 30° C. and 85% RH was used as the adhesive.

COMPARATIVE EXAMPLE 1

A BGA semiconductor device was obtained in the same manner as in Example3 except that an adhesive (available from Hitachi Chemical Co., Ltd.;trade name: EN-4500) having a curing start temperature (Th) above 130°C. was used as the adhesive.

Physical properties, resistance to reflow cracking, and warpagedeformation of Examples and Comparative Examples were examined to obtainthe results shown in Table 2.

Here, the curing start temperature (Th) of the adhesive was measuredwith a differential scanning calorimeter (manufactured by TA InstrumentsJapan Inc.; trade name: Model 910) at a heating rate of 10° C./minute.The glass transition temperature of the organic insulating encapsulantwas measured with a thermomechanical analyzer (manufactured by RigakuInternational Corporation; trade name: TMA-8141BS, TAS-100).

Evaluation on the resistance to reflow cracking and warpage deformationwas made in the following way.

(1) Resistance to Reflow Cracking:

The semiconductor device was moistened for a stated time underconditions of 85° C. and 60% RH, and thereafter reflow-treated threetimes at 240° C. for 10 seconds in an infrared reflow furnace. Any voidsand cracking or separation of the adhesive layer, having-occurred in theinterior of the semiconductor device were observed with an ultrasonicinspection device. Those in which the cracking or separation extended upto the gold-plated-terminal regions were regarded as defectives (numberof defectives was written as a numerator, and number of test devices asa denominator).

(2) Warpage Deformation:

Measured at the diagonal-line central part on the back of asemiconductor device standing after mount of the semiconductor elementand before mount of the solder balls. In the room temperature region,values at 25° C. were measured with a surface profile analyzer, and, inthe molding temperature region, values at 175° C. were measured with anon-contact laser analyzer. Then, an average value (the number ofsamples: 4) of maximum values of the extent of deformation on the basisof that of the outermost external connecting terminals was shown as theextent of warpage deformation.

As can be seen from Table 2, the semiconductor devices of Examples 1 to5 all did not cause any voids in their adhesive layers. In particular,Examples 1 to 3 showed a remarkable improvement in the resistance toreflow cracking. On the other hand, in Comparative Example 1, making useof an adhesive having a high curing start temperature (Th), voidsoccurred in the adhesive layer.

In Examples 1 and 2, making use of suitable encapsulants, not only anyvoids did not occur in the adhesive layer, but also the warpagedeformation at both 25° C. and 175° C. was able to be kept at a smallextent. More specifically, in these Examples, a high resistance toreflow cracking was achievable on account of a low moisture absorptionof the organic insulating encapsulant, and a small extent of warpagedeformation was achievable on account of the glass transitiontemperature of the organic insulating encapsulant, the difference in thecoefficient of linear expansion between the encapsulant and thesemiconductor-element-mounting substrate, the modulus in flexure atmolding temperature and the curing shrinkage factor which were setappropriate, thereby obtaining good semiconductor devices whichsatisfied the standard of within plus-minus 150 μm prescribed in JEDEC(Joint Electron Device Engineering Council).

POSSIBILITY OF INDUSTRIAL APPLICATION

As described above, the present invention makes it possible to provide asemiconductor device that may less cause cracking at the time of solderreflowing. It also makes it possible to provide an OMPAC type BGApackaged semiconductor device that can prevent occurrence of warpagedeformation. Hence, the use of the semiconductor device of the presentinvention enables reduction of faulty electrical connection, promising agreat industrial value.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor-element-mounting substrate having an insulating basematerial on which a wiring pattern has been formed; a semiconductorelement having an electrode, and bonded to thesemiconductor-element-mounting substrate with an adhesive; saidelectrode being connected electrically with said wiring pattern; and anorganic insulating encapsulant encapsulating at least said electrode;wherein said organic insulating encapsulant has the followingproperties: a glass transition temperature between molding temperatureand room temperature; a difference of 0.6×10⁵/° C. or above between thecoefficient of linear expansion of said semiconductor-element-mountingsubstrate and the coefficient of linear expansion of said organicinsulating encapsulant at a temperature not higher than the glasstransition temperature; a curing shrinkage factor of 0.11% or less atthe time of the molding of said organic insulating encapsulant; asaturation moisture absorption of 0.36% by weight or less at 85° C. and85% RH; and a flexural modulus of 4.0 GPa or below at the moldingtemperature of the organic insulating encapsulant; said adhesive havingan exothermic-reaction curing start temperature of 130° C. or below asmeasured with a differential scanning calorimeter at a heating rate of10° C./minute, and said adhesive having a saturation moisture absorptionof 0.18% by weight or less at 30° C. and 85% RH.
 2. The semiconductordevice according to claim 1, wherein; saidsemiconductor-element-mounting substrate has an external connectingterminal conducting to the wiring pattern, formed on the back which isopposite to the side on which the wiring pattern has been formed; andsaid organic insulating encapsulant so encapsulates the semiconductorelement that a surface of said semiconductor element is not uncovered.3. A process for fabricating a semiconductor device, comprising thesteps of; bonding with an adhesive a semiconductor element to thesurface of a semiconductor-element-mounting substrate having aninsulating base material on which a wiring pattern has been formed, toconnect an electrode of the semiconductor element electrically with thewiring pattern; and encapsulating at least said electrode of thesemiconductor element with an organic insulating encapsulant; whereinsaid organic insulating encapsulant has the following properties: aglass transition temperature between molding temperature and roomtemperature; a difference of 0.6×10⁵/° C. or above between thecoefficient of linear expansion of said semiconductor-element-mountingsubstrate and the coefficient of linear expansion of said organicinsulating encapsulant at a temperature not higher than the glasstransition temperature; a curing shrinkage factor of 0.11% or less atthe time of the molding of said organic insulating encapsulant; asaturation moisture absorption of 0.36% by weight or less at 85° C. and85% RH; and a flexural modulus of 4.0 GPa or below at the moldingtemperature of the organic insulating encapsulant; said adhesivecomprising an adhesive having an exothermic-reaction curing starttemperature of 130° C. or below as measured with a differential scanningcalorimeter at a heating rate of 10° C./minute and a saturation moistureabsorption of 0.18% by weight or less at 30° C. and 85% RH.